1. Field of the Invention
The present invention relates to oxygen concentration detection using an oxygen concentration sensor of a limit current type. The present invention may be used to detect an air-fuel ratio of air-fuel mixture from an oxygen concentration in exhaust gas of an engine.
2. Description of Related Art
A conventional air-fuel ratio detecting apparatus for instance, U.S. Pat. No. 4,224,113 (JP-A 55-62349) shown in FIG. 12 uses an oxygen concentration sensor as an air-fuel ratio sensor. In FIG. 12, an air-fuel ratio sensor 80 to be arranged in an exhaust pipe of a gasoline engine comprises: a diffusion resistance layer 81 exposed to an exhaust gas; a solid electrolyte layer 82 made of a known oxygen ion conductive oxide; electrodes 83a, 83b fixed to both faces of the solid electrolyte layer 82; a partition 85 for forming an atmosphere chamber 84; and a heater 86 embedded in the partition 85. In this sensor, the electrode 83a corresponds to an exhaust-side electrode and the electrode 83b corresponds to an atmosphere-side electrode.
FIGS. 13A and 13B show the fundamental principle of air-fuel detection by the air-fuel ratio sensor 80. Basically, a voltage Vp is applied across the electrodes 83a and 83b from a power source 87 and the value of a current flowing in this instance is detected, thereby detecting the air-fuel ratio. That is, when the air-fuel ratio is on the lean side (less fuel in air-fuel mixture supplied to an engine and more oxygen in the exhaust gas from the engine), as shown in FIG. 13A, the air-fuel ratio sensor 80 takes oxygen from the exhaust. In this instance, oxygen ions (O.sup.2-) flow from the electrode 83a toward the electrode 83b. When the air-fuel ratio is on the rich side (more fuel in air-fuel mixture supplied to the engine and less oxygen in the exhaust gas from the engine), as shown in FIG. 13B, the air-fuel ratio sensor 80 takes unburned gas components such as CO from the exhaust. In this instance, oxygen ions (O.sup.2-) flow from the electrode 83b to the electrode 83a. That is, the direction of oxygen ions flowing in the solid electrolyte layer 82 when the air-fuel ratio is on the rich side is opposite to that when the air-fuel ratio is on the lean sides. A limit current value Ip is positive at the time of the lean side and is negative at the time of the rich side.
FIG. 14 shows a V-I (voltage-current) characteristic of the air-fuel ratio sensor 80 having the above construction. FIG. 14A shows a V-I characteristic under a predetermined condition (lean). As shown in the diagram, in a zone where an applied voltage is low, a resistance characteristic of the solid electrolyte layer 82 is detected and the V-I characteristic shows a proportional relation. In a zone where the applied voltage is high, since movement of oxygen is regulated by the diffusion resistance layer 81, the current value is constant. The constant current value corresponds the limit current value. Generally, the zone showing the resistance characteristic is called a resistance dominating zone and the zone indicating the limit current value is called a limit current zone.
FIGS. 14B and 14C are characteristic diagrams showing change in the limit current value Ip corresponding to the change in the air-fuel ratio (A/F). According to FIG. 14B, as the air-fuel ratio changes, the limit current value Ip changes. It is understood that the more the air-fuel ratio shifts to the lean side, the larger the Ip value becomes. As shown by a characteristic line L1 of FIG. 14C, it is understood that since the relation between the air-fuel ratio and the limit current value Ip is a one-to-one corresponding relation, the air-fuel ratio can be detected from the limit current value Ip.
In the conventional air-fuel ratio detecting apparatus, however, when the air-fuel ratio is on the rich side, the following problem occurs. Although it is necessary to take oxygen from the atmosphere chamber 84 as shown in FIG. 13B when the air-fuel ratio is on the rich side, if the degree of richness is high, a large amount of oxygen is necessary. In such a case, the supply of oxygen from the atmosphere to the atmosphere chamber 84 is insufficient so that oxygen becomes deficient in the atmosphere chamber 84. When oxygen is deficient in the atmosphere chamber 84, oxygen does not flow any further, so that the limit current value Ip cannot be accurately detected. Consequently, as shown by a characteristic line L2 in FIG. 14C, the detection accuracy of the air-fuel ratio deteriorates when the air-fuel ratio is on the rich side. This may be caused because a passage to take atmosphere into the atmosphere chamber 84 is generally long and the atmosphere chamber 84 cannot be enlarged more than needed in order to secure a temperature increasing characteristic of the element by the heater.
When the air-fuel ratio is on the lean side, the following problems occur by similar reasons. That is, when the air-fuel ratio is on the lean side, as shown in FIG. 13A, oxygen is sent from the exhaust gas to the atmosphere chamber 84 on the contrary to the case where the air-fuel ratio is on the rich side. Consequently, when the degree of leanness is high, a large amount of oxygen flows into the atmosphere chamber 84. In such a case, the exhaust of oxygen from the atmosphere chamber 84 to the outside becomes insufficient, so that oxygen becomes excessive in the atmosphere chamber 84. When oxygen is excessive in the atmosphere chamber 84, the amount of oxygen passing through the solid electrolyte to the atmosphere chamber 84 is regulated, so that the limit current value Ip cannot be accurately detected. Consequently, as shown by a characteristic line L3 in FIG. 14C, the detection accuracy of the air-fuel ratio deteriorates when the air-fuel ratio is on the lean side.
On the other hand, according to U.S. Pat. No. 4,882,033 (JP-B2 4-73101), in order to cope with the problems due to oxygen deficiency, power supply unit for supplying oxygen into an atmosphere chamber (reference gas atmosphere) is provided. However, when the amount of oxygen flowing in the solid electrolyte layer changes due to change in the air-fuel ratio or the element temperature, the amount of oxygen in the atmosphere chamber cannot be maintained to a constant amount. Accordingly, the limit current value cannot be accurately detected and the air-fuel ratio detection accuracy deteriorates.
In U.S. Pat. No. 4,784,743 (JP-A 61-134656), an air-tight space communicating with an atmosphere chamber (gap) is provided so as to absorb fluctuation in the concentration of oxygen in the atmosphere chamber. Since the size of the atmosphere chamber is limited, the supply of oxygen from the air-tight space to the atmosphere chamber is insufficient. When the air-fuel ratio or the element temperature largely changes, the air-fuel ratio detection accuracy deteriorates. Although a partial voltage value of a heater voltage having a positive temperature characteristic is applied to oxygen supply unit so as to adjust an oxygen supply amount to the atmosphere chamber in accordance with a temperature change, the value to be applied depends on both of temperature characteristics of a heater resistor and an element resistor. Consequently, the oxygen amount depending only on the temperature characteristic of the element resistor cannot be controlled to be constant.
Consequently, in the conventional apparatuses shown in U.S. Pat. No. 4,882,033 and U.S. Pat. No. 4,784,743, the concentration of oxygen in the atmosphere chamber cannot be properly kept and the air-fuel ratio detection accuracy deteriorates when the air-fuel ratio or the element temperature changes.